This invention relates to an antenna structure, and, more particularly, a novel conformal aerodynamic antenna structure having broadband characteristics as well as a radiation pattern and impedance characteristics that are essentially independent of frequency over a wide range.
In designing antenna structures, it should be kept in mind that the antenna designer must make the antenna perform a desired electrical function such as transmitting/receiving linearly polarized, righthand circularly polarized, left-hand circuitry polarized, etc., r.f. signals with appropriate gain, bandwidth, beamwidth, minor lobe level, radiation efficiency, aperture efficiency, receiving cross section, radiation resistance and other electrical characteristics. It is also necessary for these structures to be lightweight, simple in design, inexpensive and unobtrusive since an antenna is often required to be mounted upon or secured to a supporting structure or vehicle such as high velocity aircraft, missiles, and rockets which cannot tolerate excessive deviations from aerodynamic shapes. Of course, it is also sometimes desirable to hide the antenna structure so that its presence is not readily apparent for aesthetic and/or security purposes. Accordingly, the ideal electrical antenna should physically be small in volume and not protrude on the external side of a mounting surface, such as an aircraft skin or the like, while yet still exhibiting all the requisite electrical characteristics.
In designing antenna structures, the environment in which they are to effectively operate must be kept in mind. For example, when such antenna structures are placed on aircraft and/or missiles, they must exhibit mechanical characteristics to enable them to withstand extreme thermal environments without degradation in electrical performance. In this regard, previous approaches have been to use high temperature material as an antenna radome and attempt to tune such antenna structures after installation. As a result, this procedure does not fully lend itself to inexpensive high volume production due to the level of skill which is required in properly tuning such antenna structures.
Antennas that have very low profiles which may be flush mounted on supporting surfaces are generally referred to as conformal antennas. As discussed, these antennas must actually conform to the contour of the supporting surface, and, therefore, reduce or eliminate any turbulent effects that would result when such devices are mounted or secured to a vehicle and propelled through space. Conformal antennas may, of course, be constructed by several methods, but can be generally produced by rather simple photoetching techniques since such techniques offer ease of fabrication at a relatively low production cost.
Such conformal antennas or printed circuit board antennas, as they may be called, are formed by etching a single side of a unitary metallically clad dielectric sheet or electrodeposited film using conventional photoresist-etching techniques. Typically, the entire thickness of antenna structure may possibly be at some fraction of a wavelength and be made to minimize cost and maximize manufacturing and/or operating reliability and reproducibility. It can be appreciated that fabrication cost may be substantially minimized since single antenna elements and/or arrays of such elements together with appropriate r.f. feedlines, phase shifting circuits and/or impedance matching networks may all be manufactured as integrally formed electrical circuits along using low cost photoresist-etching processes commonly used to make electronic printed circuit boards. This is to be compared with many complicated and costly prior art techniques for achieving polarized radiation patterns with internal but separate component fabrication as, for instance, a turnstile dipole array, the cavity backed turnstile slot array and other types of special antennas.
A resonant antenna is one which is an integral number of half-wavelengths. In a resonant antenna standing waves of current and voltage are established causing the maximum amount of radiated energy to be radiated as the antenna reactance for a particular frequency is lowest. A common example of a resonant antenna is the long open-ended linear antenna in which there is a sinusoidal current distribution having two waves of equal amplitude and 180.degree. phase difference at the open-end traveling in opposite directions along its length. The voltage distribution has also a standing wave pattern except that it has maxima at the end of the line instead of nulls as the current. For such distributions, the maxima and minima repeat every integral number of half-wavelengths. In such a distribution there is a one-quarter spacing between a null and maximum in each pattern. Thus, a resonant antenna may be referred to as a standing wave antenna.
Of course, an antenna need not exhibit resonant properties to operate satisfactorily. An antenna may operate and be designed to have approximate uniform current and voltage amplitudes along its length. Such an antenna is generally characterized as a traveling wave antenna and is nonresonant. This may be accomplished by properly terminating the antenna structure so that reflections are substantially reduced. Usually a progressive phase pattern is associated with the current and voltage distribution for such traveling wave or nonresonant antennas. Polyrod, helix, long wires, Yagi-Uda, log-periodic and slots and holes in a waveguide as well as numerous aperture antennas including reflectors and horns are typical illustrations of discrete-element traveling wave antennas.
In general, an antenna is limited in the range of frequencies over which it effectively operates. An antenna may operate satisfactory, of course, within a fixed frequency range with a signal that is yet narrower in its bandwidth and, generally, in the design of such an antenna there are no particular bandwidth problems. On the other hand, if a broadband antenna is required, there are often a number of difficulties that an antenna designer must overcome to produce a satisfactory operating antenna device. Under certain conditions, it is possible in a number of applications to actually use an essentially narrow-band antenna over a wide frequency range if allowance and provisions are actually made for modifying the antenna's dimensional characteristics or for adjusting the impedance matching transformer characteristics of the antenna. In many operations, however, it is necessary that an antenna structure having a fixed configuration operate over a very broad range of frequencies. Accordingly, a number of broadbanding techniques have been practiced to achieve this operating condition since an antenna having a broad bandwidth is highly desirable.
In considering bandwidth, there are generally two categories of parameters to be addressed: (1) the antenna radiation pattern, and (2) impedance characteristics. As regards the radiation pattern, parameters to be considered for designing a broadband antenna include the power gain, beamwidth, side-lobe level, beam direction and polarization and, as regards the impedance characteristics, parameters to be considered include input impedance, radiation resistance and antenna efficiency.
With respect to a resonant antenna, resistive loading of such an antenna provides a means to broaden its impedance bandwidth. In this regard, broadband dipole antennas have been made by making the thickness of the conducting element large relative to their length. Thus, broadbanding dipole structures have been simply accomplished by employing large diameter conductors rather than thinner ones. In this regard, biconical antennas belong to this general class and are generally considered to be broadband antennas. Nonetheless, resistive loading is not generally employed for antennas operating at high frequencies since conductor losses are usually exceeding small which, in turn, results in an antenna having an inadequate bandwidth.
Certain antennas having a wide variety of physical sizes and shapes are known to be frequency independent, often achieving bandwidths of at least 10 to 1 and substantially higher. In general, their broadband behavior includes both impedance and radiation pattern characteristics. Such frequency independent antennas, as they are called, generally exhibit a certain shape or pattern of geometric form. For such antennas there are certain structural patterns that are more or less repeated with changing dimensions. An illustrative example of this design characteristic is found in the so-called log-periodic dipole array antenna.
Although a number of such antennas are known and include the Beverage antenna, spiral antennas, rhombic antennas, the biconical and the aforementioned log-periodic antennas, all these devices are relatively large and require substantial space. Further, such antennas do not lend themselves to flush or low-silhouette installation.
The subject invention relates to an easy to fabricate conformally mounted antenna that offers a number of outstanding advantages over the prior art. Moreover, although certain related patents may at first blush appear to be closely related to the subject invention, closer inspection reveal significant differences. In this regard, U.S. Pat. No. 3,868,694 to Meinke relates to a dielectric directional antenna that uses a wedge shaped dielectric with conducting exciters on each side of the angular sides. In one embodiment of Meinke, one of the exciters can be triangular and the other a ground plane with a coaxial line feeding the exciters. U.S. Pat. No. 3,099,836 to Carr discloses a V-strip antenna with an artificial dielectric lens in which the strips may have a parabolic curvature. In the Carr antenna, the bandwidth is extended to higher frequencies operable over a frequency range of greater than 10 to 1 by the use of metal elements concentrated and arranged between the inside surfaces of his antenna. Further, U.S. Pat. No. 2,822,542 to Butterfield discloses a directive antenna device in which a waveguide may be coupled to a dielectric radiating member having a wedge shaped configuration. It should be noted that in the antenna as disclosed by Butterfield, the bandwidth of his antenna would be limited by the cutoff frequency of the waveguide feed.
In the prior art, flush mounted channel guide antennas are known as a surface wave structures consisting of a solid dielectric waveguide of rectangular cross section embedded in a metallic channel with one end of the dielectric and channel tapered to launch the radiation efficiently at the end of the dielectric. The other end of the channel may be connected to the r.f. source by various means to launch a surface wave in the channel. Typically, the launcher might be an open end waveguide, horn, or wire launcher.
Although the aforementioned representative patents have certain characteristics in common with the subject invention, they do not demonstrate the advantageous structural characteristics and associated broadband properties as disclosed and claimed herein.